In the field of high-power lasers having large beam diameters, it is common for the quality of the optical beam emerging from the laser gain medium to be inadequate for the purposes on hand, especially since the wave front is not ideal, that is, phase shifts exist between different regions of the cross section of the laser beam. To deal with this situation, it has already been proposed to aim the emerging laser beam against a reflecting surface of a thin flexible mirror faceplate for the laser beam to be reflected by the reflective surface, and to adjust the contour or shape of the thin flexible plate by means of one or more actuators which extend forward and backward and thus change the shape and contour of the flexible plate and thus the contour of the reflective surface of the mirror. The deformable mirror can be used to alter the wave front of the laser beam to allow the laser beam to be propagated more easily through the medium into which it is ultimately injected for propagation, such as through the atmosphere.
In order to be able to correct for optical phase distortions to a high energy laser (HEL) beam as it is propagated through the atmosphere, it has been established that there is a pronounced need for the use of deformable mirrors acted upon and deformed by closely packed actuators, which may have to be spaced at less than 1 centimeter apart. With the advent of more powerful HELs, the beam size, of necessity, increases, resulting in the need for using very large numbers of closely packed actuators, and in the need for using active cooling to remove absorbed heat energy from the mirror.
The closely packed actuator array causes difficulties in the employment of replaceable actuator design features. This is so because the required small spacing of the actuators does not offer reasonable sizing of the necessary mechanical features as executed in state-of-the-art designs. Moreover, heretofore known replaceable actuator designs are known to possess deficiencies with respect to current and future needs, particularly for short wavelength devices.
So, for instance, the force train mechanism of the actuator arrangement has many components whose alignment and fabrication accuracy are critical to proper operation. In conventional constructions of deformable mirrors in which the actuators of the actuator arrangement act on the mirror faceplate through respective actuator pads formed on the rear side of the faceplate, in which retracting springs act on the faceplate in the rearward direction, and in which a heat exchanger or cooling chamber is situated behind the faceplate, actuator replacement may result in unacceptable mirror shape degradation because of the force train variations which occur as a result of such replacement. In such known arrangements, even small force train misalignments result in the application of unacceptable moments to the heat exchanger, and these moments then cause distortion of the reflective surface of the faceplate. The actuator to faceplate interface is critical because of the faceplate and reflective surface sensitivity to bending moments. Such bending moments result from tolerance errors and they cause a misalignment in the respective path in which the actuator load is applied to the respective actuator pad of finite height, where the bending moment is proportional to the pad height. Such misalignment may be the result of eccentricity and squareness tolerance, retraction spring offset, contour mismatch in spherical joints, and "walking" (non-slip) in spherical joints during operation.
Moreover, during actuator removal and installation, the full force of the retraction spring is applied to the heat exchanger. The force applied by the retraction spring is usually sufficient to cause intolerable stresses in the heat exchanger of the finished mirror, and could produce unwanted reflective surface degradation. Also, binding and galling of the threads of the actuator assembly could present actuator removal difficulties. Thread backlash, roughness and friction characteristics affect the adjustment resolution of the respective actuator assembly. Mechanical locking of threads imposes strains that shift the adjusted surface.
Another disadvantage of known constructions of the cooled flexible mirror assemblies is that the mirror heat exchanger designs that have been heretofore used are not compatible with the currently required close packing of the actuators and with large mirror apertures. Large differential strokes between adjacent actuators cannot be tolerated in the applications with which the present invention is concerned while using conventional channel flow heat exchangers because flexural stiffness and resulting stress levels are too high. Moreover, large aperture designs either require high pressure loss (for a single zone), which results in high stresses, or must accommodate multiple cooling zones, which results in mechanically complex interfaces between the coolant supply and the heat exchanger.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a mirror arrangement which does not possess the disadvantages of the known mirror arrangements of this kind.
Still another object of the present invention is so to develop the mirror assembly of the type here under consideration as to be compatible with very close packing of the actuators and relatively large surface deformations between adjacent actuators.
It is yet another object of the present invention to devise a mirror arrangement of the above type which reduces tolerance effects on and particularly the application of bending moments to the faceplate carrying the reflective surface of the mirror to a minimum, if not eliminating them altogether.
An additional object of the present invention is to design the above mirror arrangement in such a manner as to provide low thermal distortion and low jitter characteristics, and to be scalable to very large sizes without performance loss or geometry changes.
A concomitant object of the present invention is construct the mirror arrangement of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.